My mini does just that and has changed my life. Try to find the older firewire based eyetv 200 box for it. If you want to pick my brain as to how mine is set up feel free.
Same is true of the RAM slots in a MacBook Pro. And the Intel Mini has double the slots of the G4. Plus if you do the CPU, you might find the RAM limit goes up. Not sure of this though. You can now OC the GMA950 too.
I had the first generation mac mini with the core solo in it. It was an awesome machine, and so compact. Downside was the DVD drive went out in it and its quite the hassle to take apart. Fully support those machines though. I'd try and steer towards a core solo machine though.